snmpit_force10: implement trunking functions; refactor and cleanup.
snmpit_force10: implement trunking functions; refactor and cleanup.

Added port trunking methods to the snmpit_force10 module, but
stopped short of adding port channel (LAG) functionality.  The latter
requires support from either a future REST API, or a somewhat hokey
HTTP-RPC mechanism that invokes switch-side scripts.

Also did some cleanup and minor refactoring.

nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome